Reponse: Fix incomplete implementation in write

Write function in http_response.hpp missing implementation for first
write after changing from filebody.
Usecase:
Current resonse type is filebody. Developer tries to change the body
type to stringbody by calling write function.
Observed:
The write fails to update the body type.
Expected:
Write should succeed and body should change to string body.

Tested:
Unit test has been added for crow::Response.
Manual sanity test done for file offloads using curl.

Change-Id: Icbf8585b5b04c3ac5120d7b334c13d89ed3eb4aa
Signed-off-by: Abhilash Raju <abhilash.kollam@gmail.com>
diff --git a/meson.build b/meson.build
index f3e0ed3..002b949 100644
--- a/meson.build
+++ b/meson.build
@@ -409,6 +409,7 @@
   'test/http/crow_getroutes_test.cpp',
   'test/http/http_connection_test.cpp',
   'test/http/router_test.cpp',
+  'test/http/http_response_test.cpp',
   'test/http/utility_test.cpp',
   'test/http/verb_test.cpp',
   'test/include/dbus_utility_test.cpp',